Traveling-wave tubes or klystrons or the like are electron tubes used to perform amplification, oscillation or the like of a high-frequency signal through interaction between an electron beam emitted from an electron gun and a high-frequency circuit. As shown, for example, in FIG. 1, traveling-wave tube 1 is constructed of electron gun 10 that emits electron beam 50, helix electrode 20, which is a high-frequency circuit that causes electron beam 50 emitted from electron gun 10 to interact with a high-frequency signal (microwave), collector electrode 30 that captures electron beam 50 emitted from helix electrode 20 and anode electrode 40 that leads out electrons from electron gun 10 and guides electron beam 50 emitted from electron gun 10 into spiral helix electrode 20.
Electron gun 10 is provided with cathode electrode 11 that emits thermal electrons and heater 12 that gives thermal energy for emitting thermal electrons to cathode electrode 11.
Electron beam 50 emitted from electron gun 10 is accelerated by a potential difference between cathode electrode 11 and helix electrode 20, introduced into helix electrode 20 and travels through helix electrode 20 while interacting with a high-frequency signal inputted from one end of helix electrode 20. Electron beam 50 which has passed through helix electrode 20 is captured by collector electrode 30. In this case, a high-frequency signal which has been amplified by the interaction with electron beam 50 is outputted from the other end of helix electrode 20.
Power supply apparatus 60 supplies a helix voltage (H/K), which is a negative DC voltage, to cathode electrode 11 using a potential (HELIX) of helix electrode 20 as a reference and supplies a collector voltage (COL), which is a positive DC voltage, to collector electrode 30 using the potential (H/K) of cathode electrode 11 as a reference. Furthermore, power supply apparatus 60 supplies a heater voltage (H), which is a negative DC voltage, to heater 12 using the potential (H/K) of cathode electrode 11 as a reference. Helix electrode 20 is normally connected to a case of traveling-wave tube 1 and grounded.
FIG. 1 shows a configuration example of traveling-wave tube 1 provided with one collector electrode 30, but traveling-wave tube 1 may also have a configuration provided with a plurality of collector electrodes 30.
Furthermore, FIG. 1 shows the configuration of a high-frequency circuit system in which anode electrode 40 and helix electrode 20 are connected in power supply apparatus 60 and a ground potential is supplied to anode electrode 40, but a voltage different from the potential of helix electrode 20 may also be supplied to anode electrode 40 individually. In that case, an anode voltage (ANODE), which is a positive DC voltage, is supplied to anode electrode 40 using the potential (H/K) of cathode electrode 11 as a reference.
The helix voltage (H/K), collector voltage (COL) and heater voltage (H) are generated using, for example, a transformer, an inverter connected to a primary winding of the transformer that converts a DC voltage supplied from outside to an AC voltage and a rectification circuit that converts an AC voltage outputted from a secondary winding of the transformer to a DC voltage.
However, since electrons emitted from cathode electrode 11 due to a potential difference between anode electrode 40 and cathode electrode 11 in traveling-wave tube 1, it is desirable to reduce the potential difference between anode electrode 40 and cathode electrode 11 as much as possible when the supply voltage to each electrode is unstable as in the case of a rise (application) of helix voltage (H/K) or collector voltage (COL). When there is a potential difference between anode electrode 40 and cathode electrode 11 when the helix voltage (H/K) and collector voltage (COL) are applied, some of electrons emitted from cathode electrode 11 flow to the ground potential through helix electrode 20, and therefore an overcurrent flows through helix electrode 20, causing characteristic deterioration or damage of traveling-wave tube 1. Especially, in the configuration as shown in FIG. 1 in which anode electrode 40 is connected to helix electrode 20, a potential difference is produced between anode electrode 40 and cathode electrode 11 simultaneously with the application of the helix voltage (H/K), and therefore it is desirable to reduce this potential difference using some means.
To avoid such a problem, for example, Japanese Patent Laid-Open No. 2005-093229 (hereinafter referred to as “Patent Document 1”) describes a configuration for controlling the supply and cutoff of an anode voltage through a circuit using an FET (Field Effect Transistor).
FIG. 2 is a block diagram showing a configuration of the high-frequency circuit system described in Patent Document 1.
As shown in FIG. 2, the high-frequency circuit system described in Patent Document 1 is provided with transistor Q1, a source of which is connected to a cathode electrode of traveling-wave tube 1, a drain of which is connected to an anode electrode and a helix electrode via resistor R1 of traveling-wave tube 1 and transistor Q2 for controlling ON/OFF of transistor Q1. An N-channel junction type FET is used for transistor Q1 and an N-channel MOSFET is used for transistor Q2.
The gate of transistor Q1 is connected to the drain of transistor Q2 and resistor R2 is connected in parallel between the gate and the source of transistor Q1. The source of transistor Q2 is connected to the heater of traveling-wave tube 1 and a voltage resulting from dividing the voltage between the helix electrode and the heater of traveling-wave tube 1, between resistors R3 and R4, is applied to the gate of transistor Q2.
In such a configuration, transistor Q1 turns ON and the potential of the anode electrode (A) substantially matches the helix voltage (H/K) for a period during which the helix voltage (H/K) and collector voltage (COL) are rising and when the helix voltage (H/K) and collector voltage (COL) rise to a certain degree, transistor Q1 turns OFF and the potential of the anode electrode (A) becomes substantially equal to the ground potential (HELIX). Timing at which transistor Q1 turns from ON to OFF is determined by the ratio of divided voltages of resistors R3 and R4 connected to the gate of transistor Q2.
In the high-frequency circuit system shown in FIG. 2, only a minimal current (IHELIX) flows through the helix electrode of traveling-wave tube 1 when transistor Q1 turns from ON to OFF as shown in FIG. 3 making it possible to prevent any overcurrent from flowing through the helix electrode, thus causing characteristic deterioration or damage of traveling-wave tube 1. The anode voltage (ANODE) shown in FIG. 3 shows a potential difference from the helix voltage (H/K) and does not show an actual voltage variation.
Furthermore, other techniques for reducing a potential difference between the anode electrode and cathode electrode when the helix voltage (H/K) and collector voltage (COL) are applied include those described in Japanese Utility Model Laid-Open No. 57-186966, Japanese Utility Model Laid-Open No. 61-157251 and Japanese Utility Model Laid-Open No. 04-076240. These publications describe a configuration in which resistors R11 and R12 are connected in series between a helix electrode and a cathode electrode of traveling-wave tube 1, a helix voltage (H/K) is divided between resistors R11 and R12 and the resulting voltage is supplied to an anode electrode (A).
FIG. 4 is a block diagram showing a configuration of a high-frequency circuit system in which the voltage divided between resistors is supplied to the anode electrode.
In the configuration shown in FIG. 4, the potential difference between the anode electrode and the cathode electrode becomes smaller compared to the configuration shown in FIG. 1 in which the anode electrode is connected to the helix electrode, and it is thereby possible to reduce the current that flows through the helix electrode when the helix voltage (H/K) and collector voltage (COL) are applied.
However, according to the configuration shown in FIG. 2, in the above described power supply apparatuses, when transistor Q1 used to control the supply and cutoff of the anode voltage is OFF, that is, when traveling-wave tube 1 is operating normally, if a current flows through the anode electrode, a current also flows through resistor R1 connected between the helix electrode and the drain, which results in a problem in which the potential of the anode electrode decreases (approximates to the helix voltage (H/K)) and a maximum gain of traveling-wave tube 1 decreases.
For example, assuming that the current flowing through the anode electrode in the normal operation of traveling-wave tube 1 is 0.1 mA and that the value of resistor R1 is 10 MΩ, the potential of the anode electrode decreases by the order of 1 KV with respect to the potential of the helix electrode. When the value of resistor R1 is reduced, the potential difference between the anode electrode and the helix electrode in normal operation decreases. In such a case, however, the helix voltage (H/K) is applied when transistor Q1 is ON and power consumption of resistor R1 increases, and therefore the size of the package of resistor R1 increases.
Since the helix voltage (H/K) of traveling-wave tube 1 is generally several KV to several tens of KV, when, for example, the helix voltage (H/K) is 10 KV and the value of resistor R1 is 10 MΩ, power consumed by resistor R1 is 10 W. Reducing the value of resistor R1 causes the power consumed by resistor R1 to further increase and thereby further increases the size of the package of resistor R1.
Furthermore, according to the configuration shown in FIG. 2, since transistor Q1 used for supplying and cutting off the anode voltage operates at a high voltage using the helix voltage (H/K) as a reference, when it is desired to control ON/OFF of transistor Q1 using a logic circuit operating at a low voltage of, for example, several V instead of using transistor Q2, it is necessary to insulate the logic circuit from transistor Q1 using a high-pressure vacuum relay or the like. In such a case, the high-pressure vacuum relay is very expensive and the cost of the high-frequency circuit system increases.
On the other hand, the high-frequency circuit system shown in FIG. 4 can reduce the current that flows through the helix electrode when the helix voltage (H/K) and collector voltage (COL) are applied compared to the configuration in which the anode electrode of traveling-wave tube 1 shown in FIG. 1 is connected to the helix electrode as described above.
However, even in the configuration shown in FIG. 4, the potential difference between the anode electrode and cathode electrode increases as the helix voltage (H/K) increases, as shown in FIG. 5, and therefore a greater current (IHELIX) flows through the helix electrode compared to the configuration shown in FIG. 2. The anode voltage (ANODE) shown in FIG. 5 shows a potential difference from the helix voltage (H/K) and does not show an actual voltage variation.
Furthermore, in the configuration shown in FIG. 4, a voltage closer to the helix voltage (H/K) than that in the configuration shown in FIG. 1 is applied to the anode electrode in normal operation, and therefore there is also a problem that the anode voltage drops in the normal operation of traveling-wave tube 1 in the same way as in the configuration shown in FIG. 2.